Twenty Fifteen automatically adds descriptions to menu items. Making this an option to enable/disable is much more user-friendly.
Where to Find it?
Customizer > Navigation > “Display Archive Descriptions in Navigation?”
The first time I installed Twenty Fifteen I noticed that it automatically added descriptions in the menu. If you only have Posts or Pages in your menu you might not notice it right away. For those you have to enable the feature in the menu area and enter your text. But Archive pages which have descriptions get added automatically. My Archives have long descriptions, so this was really impractical for my purposes. Making this a setting is much more user-friendly.